The present invention relates to programmable transversal filters (hereinafter PTF""s), particularly those using surface acoustic wave (SAW) substrates as the delay medium along with amplifiers which provide tap weight factors.
Programmable transversal filters using SAW substrates as the delay medium, which include an input interdigitated transducer and a plurality of individual output tap transducers, are well known in the art. Each of (n) tap transducers is paired with and connected to one of (n) variable gain amplifiers forming distinct signal paths for each of (n) tap transducer/amplifier combinations. The outputs of the (n) variable gain amplifiers are connected to the summing junction, which forms the output signal as the sum of the signals from the (n) signal paths. It should be noted that, in general, the signal paths should be connected to the summing junction with alternating polarity between sequential signal paths; such is readily accomplished through the use of both inverting and non-inverting inputs to an operational amplifier used as the summing junction which then provides the output of the filter. Several significant limitations are encountered with these prior art filters. Large numbers of tap transducers are very difficult to achieve thereon because the physical dimensions of the tap transducers on the substrate are not compatible with those of the amplifiers. That is, the width of an individual tap transducer along the SAW propagation direction, is substantially smaller than the width of an individual amplifier, and hence in order to connect each individual tap transducer to its corresponding variable gain amplifier, gross fan-out occurs. Consequently, the SAW device is limited to either a small number of individual tap transducers or the use of tap transducers whose widths are close to those of the variable gain amplifiers. The choice of a limited number of tap transducers results in limited passband bandwidth (passband bandwidth≈1/T) because of the short delay time (T) for the SAW to propagate through the small number of individual tap transducers on the SAW substrate.
The choice of tap transducer widths close to those of the variable gain amplifiers restricts the normal center frequency of the filter, as this frequency is determined jointly by the SAW velocity and periodic spacing of the individual tap transducers. Furthermore, the variable gain of the amplifiers must be calibrated over their full dynamic range if the operational uniformity and repeatability of the filter is to be maintained. This has proven to be impractical, particularly when environmental effects such as temperature changes are encountered. The restrictions on the number and/or spacing of individual transducers seriously limits the filter""s operational flexability. Of course, such limitations seriously affect filter quality.
It is the general object of the present invention to enhance the operational flexibility of programmable transversal filters.
It is the specific object of the present invention to achieve the general object by incorporating switch arrays into otherwise conventional circuitry.
It is a more specific object of the present invention to achieve the general object by incorporating amplifier arrays having fixed gain amplifiers, into the circuitry of the invention.
These and other objects are accomplished in accordance with the present invention, by arranging the switch arrays to permit passage for a plurality of tap voltages or signals to each amplifier.
The scope of the present invention is only limited by the appended claims for which support is predicated on the preferred embodiments hereafter set forth in the following description and the attached drawings.